1. Field of the Invention
The present invention relates to a radiography apparatus for performing tomosynthesis radiography to generate slice images of a subject.
2. Description of the Related Art
In recent years, tomosynthesis radiography was proposed to observe affected regions of patients in more detail by a radiography apparatus using X-rays. In tomosynthesis radiography, radiography is performed by moving an X-ray tube to different positions, and by irradiating a subject by the X-ray tube at different angles from the different positions. Further, images obtained by radiography are added to obtain an image in which a desired cross section of the subject is emphasized. In tomosynthesis radiography, the X-ray tube is moved parallel to an X-ray detector, or in such a manner to draw a circle or an ellipse or oval, based on the characteristic of a radiography apparatus and the kind of a slice image (tomogram) to be obtained. The subject is radiographed at different radiation angles to obtain a plurality of radiographic images, and a slice image of the subject is reconstructed from the plurality of radiographic images.
In tomosynthesis radiography, a system that can reduce a time period for obtaining a plurality of radiographic images has been proposed (please refer to U.S. Pat. No. 6,970,531 (Patent Document 1)). In the system, when a subject is irradiated with radiation, the X-ray tube is not stopped. Instead of stopping the X-ray tube, the movement speed of the X-ray is changed or the X-ray tube is continuously moved to reduce the time period for obtaining the plurality of radiographic images. Further, a system that obtains images of a plurality of cross sections of a subject in one radiography operation has been proposed (please refer to U.S. Pat. No. 7,313,219 (Patent Document 2)).
In tomosynthesis radiography, radiographic images are obtained by moving only the X-ray tube, or by moving both of the X-ray tube and the X-ray detector. To increase the size of a region in which slice images are reconstructable from a plurality of radiographic images, it is desirable that both of the X-ray tube and the X-ray detector are moved. However, when both of the X-ray tube and the X-ray detector are moved, if the X-ray detector is not moved accurately, the position of an object (target) projected onto a radiographic image shifts or the like, and there is a problem that the quality of a reconstructed slice image drops. In contrast, when only the X-ray tube is moved, the problem caused by the movement of the X-ray detector does not arises. Therefore, it is possible to reconstruct a slice image the quality of which is higher than that of a slice image obtained by moving both of the X-ray tube and the X-ray detector.
As described above, tomosynthesis radiography is performed by moving only the X-ray tube, or by moving both of the X-ray tube and the X-ray detector. Especially, the system disclosed in Patent Document 1 can optionally move the X-ray detector. Therefore, in the system of Patent Document 1, an operator (a radiographer) can select a mode in which the X-ray detector is not moved or a mode in which the X-ray detector is moved based on the condition of radiography, such as the purpose of radiography for obtaining a slice image and the size of a region of interest (ROI).
However, in the system disclosed in Patent Document 1, the operator needs to select whether only the X-ray tube is moved, or both of the X-ray tube and the X-ray detector are moved. Therefore, if the operator selects a mode that is not appropriate for the condition of radiography, it becomes impossible to obtain an image of desirable quality and a slice image including the region of interest. At this time, the operator may check a mode that is appropriate for the condition of radiography before selecting the mode. However, such operation of checking the mode every time of radiography imposes a heavy burden on the operator.